1. Field of the Invention
The present invention relates to a dry etching method employed in such applications as production of semiconductor devices. More particularly, it relates to a dry etching method whereby sulfur may be prevented from accumulating in an etching chamber to improve etching reproducibility in a sidewall protection process using sulfur deposition.
2. Description of the Related Art
The recent trend toward higher integration and performance of such semiconductor devices as VLSIs and ULSIs requires dry etching technologies to achieve correspondingly higher anisotropy, higher etchrate, higher selectivity, lower pollution, and less damage with no compromise in these requirements.
Conventionally, CFC (chlorofluorocarbon) gases typified by CFC 113 (C.sub.2 Cl.sub.3 F) or mixed gas of CFC 113 and SF.sub.6 with SF.sub.6 added to improve etchrate have been widely used to etch layers of material based on such silicons (Si) as single crystal silicon, polysilicon, refractory metal silicide, and polycide. Particularly, CFC-based gases, whose molecule contains F and Cl, allow etching using both a radical reaction and an ion-assisted reaction and sidewall protection using carbonaceous polymers deposited from the gaseous phase.
Meanwhile, CHF.sub.3 gas, mixed gas of CF.sub.4 and H.sub.2, mixed gas of C.sub.2 F.sub.6 and CHF.sub.3, and C.sub.3 F.sub.8 have been typically used to etch silicon oxide (SiO.sub.2)-based material layers. The common functions of these gases include: (a) forming a C--O bond from a constituent element C on the surface of a SiO.sub.2 layer and dissociating or weakening an Si--O bond, (b) forming CF.sub.x.sup.+ as an etchant for SiO.sub.2, and (c) generating relatively carbon-rich plasma and thereby removing oxygen from the SiO.sub.2 in the form of CO or CO.sub.2 while reducing the etchrate through carbonaceous polymers deposited on the Si and thereby achieving high selectivity for the Si.
However, CFC-based gases are commonly known to contribute to destruction of the earth's ozone layer and the production and use thereof are likely to be prohibited in the near future. In these circumstances, there is pressing need to find some appropriate substitutes for CFC-based gases for use in dry etching and establish the efficient application methods thereof.
Fluorocarbon-based gases, which are also used to etch SiO.sub.2 -based material layers, are not categorized as CFC at present but the use thereof will be restricted in the future.
In any case, the future trend toward more strict design rules for semiconductor devices may permit carbonaceous polymers deposited from the gaseous phase to become particle pollutants. In this light, too, there is pressing need to establish CFC-free etching methods.
One promising CFC-free etching method is low temperature etching, in which a radical reaction on a pattern sidewall is frozen or inhibited to avoid such etching defects as undercut with a target substrate (wafer) maintained at a temperature of 0.degree. C. or less and the vertical etchrate kept at a practical level through an ion-assisted reaction. The low temperature etching is publicized in e.g. the Extended Abstract of the 35th Spring Meeting (1988) of the Japan Society of Applied Physics and Related Societies, p.495, 28a-G-2. In this instance, a silicon trench and a n.sup.+ type polysilicon layer are etched with a wafer cooled to -130.degree. C.
However, the low temperature etching, which attempts to achieve high anisotropy only by freezing or inhibiting a radical reaction, requires considerably low temperature and may considerably reduce both economy and throughput. A more practical etching method might be to combine radical reaction inhibition with sidewall protection in low temperature to perform etching in a temperature zone close to room temperature.
The present inventor has proposed a great number of sidewall protection methods using sulfur deposition. Sulfur (S) will be deposited by etching gas when the gas is composed mainly of sulfur halides with a relatively high S/X ratio, i.e. the ratio of the number of sulfur (X) atoms to that of halogen (X) atoms.
For instance, such sulfur fluorides as S.sub.2 F.sub.2, SF.sub.2, SF.sub.4, and S.sub.2 F.sub.10 will form sulfur in the gaseous phase from dissociation thereof through electric discharge unlike more well-known sulfur fluoride SF.sub.6. When a substrate is cooled to low temperature, the sulfur thus formed will deposit on the surface thereof, producing sidewall protection effects. When the substrate is heated after completion of etching, the sulfur deposits will sublime immediately, avoiding the danger of inducing particle pollution. Noting that the above mentioned sulfur halides form a smaller amount of sulfur radical (F*) than SF.sub.6 and that SF.sub.x.sup.+ serves for an ion assisted reaction, the present inventor has proposed a method of applying these sulfur halides to etch SiO.sub.2 -based material layers to achieve high selectivity for Si underlying layer.
Thus, the present assignee has first proposed sulfur fluoride with a relatively high S/F ratio as a sulfur halide for etching SiO.sub.2 -based material layers. Further, the present assignee has later proposed various methods of applying sulfur halides to etch Si-based material layers.
An example of such etching methods is to etch Si-based material layers by using gas containing such sulfur chlorides as S.sub.2 Cl.sub.2 or such sulfur bromides as S.sub.2 Br.sub.2 with a target substrate cooled to 0.degree. C. or less. This etching method aims at reducing the effects of radicals and achieving higher anisotropy by using gas unlikely to form highly reactive F*.
The above mentioned etching method using sulfur halides is an epoch-making CFC-free etching method in that it allows clean anisotropic etching in practical temperature zones. However, the subsequent study by the present assignee has shown that practical application of the etching method requires more strict control over the S/X ratio of an etching reaction system.
In the above mentioned etching method, sulfur formed in the gaseous phase is used for sidewall protection and deposits on the inside wall of an etching chamber and the surface of various members. It has proved, however, that the sulfur thus formed will continue to deposit in a single-slice etching apparatus not at a constant rate but at widely varying rates depending on whether the number of etched wafers is large or small. Therefore, the S/X ratio of the etching reaction system will vary slightly with the number of etched wafers and then increase sharply under the strong influence of sulfur deposited on other members than the wafers when the deposition rate of the sulfur reaches an almost saturated point. In this event, the sulfur will deposit excessively on the wafers and cause such problems as pattern tapering and etchrate reduction.